Trichlorosilane (HSiCl3) has long been used as a raw material for high-purity polycrystalline silicon, which is used for the production of silicon wafers, and the like. As to the technique for obtaining trichlorosilane, many synthetic methods are known, and Japanese Patent Laid-Open No. 56-73617 (Patent Literature 1) discloses an invention relating to a method for producing trichlorosilane, advantageously characterized in that the by-product silicon tetrachloride is efficiently converted into trichlorosilane in the production of trichlorosilane.
Other known methods for producing trichlorosilane include a direct method in which metallurgical grade silicon is allowed to contact hydrogen chloride at a temperature of about 250° C. or higher (refer to Japanese Patent Laid-Open No. 2-208217 (Patent Literature 2) and Japanese Patent Laid-Open No. 9-169514 (Patent Literature 3), and the like), a method for reducing silicon tetrachloride to trichlorosilane by reacting it with hydrogen in the presence of metallurgical grade silicon (refer to Japanese Patent Laid-Open No. 60-36318 (Patent Literature 4)), a method in which, using copper silicide in place of the metallurgical grade silicon, silicon tetrachloride is reduced to trichlorosilane by reacting it with hydrogen in the presence of the copper silicide (refer to Japanese Patent Laid-Open No. 10-29813 (Patent Literature 5)), and the like.
Meanwhile, an impurity such as phosphorus or boron acts as a donor or acceptor in a silicon crystal. Thus, when these dopant components are present in the raw material polycrystalline silicon used for the production of semiconductor, they will be taken up in the final silicon wafer product. For this reason, for the production of semiconductor grade polycrystalline silicon, high-purity trichlorosilane obtained through precise distillation is used.
In connection with the these manufacturing technologies for high-purity trichlorosilane, a method for separating and removing the aforementioned dopant components in advance before distillation of trichlorosilane by converting them into an easily separable form with a getter and the like is also proposed (for example, refer to Japanese Patent Laid-Open No. 2004-250317 (Patent Literature 6)).
Further, carbon impurities in silicon crystals form an impurity level within the band gap and act as a carrier trap, accelerate the formation of precipitation nuclei of oxygen in the crystals to induce defects in the production process of semiconductor devices, and the like. In view of the above, carbon impurity content is also a problem in semiconductor grade polycrystalline silicon.
Carbon impurities contaminating polycrystalline silicon may include carbon-containing compounds derived from a carbon member used in a CVD reactor which is employed for deposition of polycrystalline silicon, carbon-containing compounds contained in trichlorosilane or hydrogen, and the like. However, it is not easy to produce trichlorosilane from which carbon-containing compounds have been sufficiently removed.
This is due to the following reasons: Because metal silicone used for direct synthesis of trichlorosilane is produced in an arc furnace using a carbon electrode, it has a purity of only about 99% and includes carbon as an impurity. Also, because the product for trichlorosilane synthesis flowing out of a CVD reactor contains methylchlorosilanes derived from the carbon member inside the CVD reactor, trichlorosilane purified by distillation still contains a trace amount of the aforementioned carbon-derived methylchlorosilanes. For example, when a carbon impurity-containing product produced in an arc furnace using a carbon electrode is used as the metallurgical grade silicon, lower boiling methylchlorosilanes will be mixed in an amount of about several tens of ppm in terms of weight ratio as the carbon impurity-derived by-product.
Particularly, removal of methyldichlorosilane is difficult because it is not only the main component of methylchlorosilanes that are mixed in as described above but also its boiling point (41° C.) is close to that of trichlorosilane (32° C.), which is a target product to be purified by distillation.